The technology in this application is described in an example Long Term Evolution (LTE) context. However, the technology can be applied to other types of networks and standards, e.g., GSM, uTRAN, etc. The architecture of the LTE system (sometimes called Evolved Universal Terrestrial Radio Access Network (E-UTRAN)) including logical interfaces between eNodeBs (eNBs) 10 (X2 interface) and between eNB 10 and Mobility Management Entity (MME)/Serving Gateway (S-GW) 12 (S1 interface) is shown in FIG. 1. LTE is based on a “flat” architecture compared to 2G and 3G systems. Each cell is associated to a frequency carrier, and is served by an eNodeB or eNB (“base station”). The E-UTRAN is made up of eNB nodes 10, which are connected to each other via the X2 interface. Both the S1 and the X2 interfaces can be divided into control plane (dashed lines) and user plane (solid lines) parts.
In addition to the user and control planes specified in 3GPP, there is an operation and management (OaM) architecture for network management to support configuration, equipment management, fault management, performance management etc. An example management system for 3GPP is shown in FIG. 2. The network elements (NE), also referred to as eNB 10, are managed by a domain manager (DM) 18, also referred to as the operation and support system (OSS). Sometimes the individual NEs, e.g., eNBs, are handled by an element manager (EM), which is a part of the DM 18. Typically, the DM 18, 20 manages only equipment from the same vendor. The DM tasks include configuration of the network elements, fault management, and performance monitoring. The latter task can require regular transfer of extensive data from events and counters from the network elements up to the DM 18. A DM may be managed by the network manager (NM) 16 via an interface called Itf-N. The NM 16 is also referred to as the Network Management System (NMS).
Two NEs 10 communicate over an X2 interface. The interface between two DMs 18 is referred to as Itf-P2P. Multi-vendor management can be handled either via the common NM and the interface Itf-N, or via the peer-to-peer interface Itf-P2P. Furthermore, the X2 interface between eNBs also supports some management, such as informing neighboring eNBs about served cells and their configurations. Because the X2 interface is standardized, eNBs from different vendors can readily communicate. Example information exchanged over X2 between peer eNBs includes served cell configuration information (carrier frequency, Cell Global ID, etc.), cell configuration for neighbor cells, signaling to request peer eNBs to modify mobility thresholds, and signaling to enable the activation of dormant cells (for energy saving) by a peer eNB.
Traditionally, a mobile network is deployed as a single service coverage layer, where the cell coverage areas are as mutually-exclusive as possible. In E-UTRAN (as well as in UTRAN), all cells can use the same carrier frequency. A heterogeneous wireless network (HetNet) augments the single layer network (sometimes referred to as the underlay layer or macro layer) with another overlay layer (sometimes referred to as a pico layer). The typical purpose of an overlay/pico layer is to provide extended capacity in a small area. This overlay/pico layer typically has for less coverage than the underlay/macro layer because it uses cells with less power and lower-mounted antennas (maybe even mounted indoors). The cells of the overlay layer/pico can use the same carrier(s) as the underlay/macro layer cells.
The underlay/macro network is typically deployed with little overlap between macro cells so that UEs typically receive little interference in the downlink (DL) from other macro base stations. Also, on the uplink (UL), the UEs typically cause little interference to other macro cells. But in the HetNet scenario, the coverage area of a pico cell is typically completely covered by a macro cell. A UE attached to a pico cell is therefore interfered in the DL by DL transmissions from the macro cell. In the UL, UEs transmitting to the pico cell also interfere with the macro cell. This interference increases if Cell Range Extension is used. Cell Range Extension artificially increases a coverage area of a pico cell.
FIG. 3 shows a graphical representation of Cell Range Extension (CRE). With CRE, a handover trigger point between a Macro cell 22 served by a Macro base station 10M and a Pico cell 24 served by a Pico base station 10P changes so that a UE 26 connected to a Macro cell 22 hands over earlier to the Pico cell 24. Conversely, with this changed handover trigger point, a UE connected to a Pico cell hands over later to the Macro cell. A UE located in the CRE area between extended and nominal range/coverage shown in FIG. 3 is subjected to higher DL interference from the Macro cell 22 and generates higher UL interference to surrounding cells due to higher path loss to the Pico cell.
There are several approaches for mitigating the interference. Although a base station transmits some symbols (pilots, sync, system information, etc.) at full power, the transmission power level for other symbols used for control and data channels can be sent with reduced power or not sent at all during limited time periods. One such technique is Almost Blank Subframes (ABS) and another is Reduced Power SubFrames (RPSFs). Both techniques are examples of protected subframes. As shown in FIG. 4, the data channel transmit power, e.g., PPDSCH, of the Macro base station in the macro cell is zero (on the left) or very low (in the middle) on certain subframes. A macro base station is either configured with an ABS power, which corresponds to blank the first two subframes as in the first graph on the left, or configured with RPSF, which corresponds to the first two subframes as in the middle graph.
An ABS pattern in LTE extends over M subframes and repeats so that subframe 1 has the same configuration as subframe M+1. In frequency division duplex (FDD) operation, M=40, and in time division duplex (TDD) operation, M=70. In other words, the ABS subframe pattern indicates which subframes among the M subframes are blanked, while the rest of the subframes are operated at full transmission power. The right-most graph in FIG. 4 shows how the pico base station behaves towards its pico cell edge UEs. If the pico base station is made aware about the ABS subframe pattern, via X2 signaling for example, the pico base station can schedule cell edge UEs to communicate during the blanked subframes, accordingly. In this example case, a cell edge UE is scheduled during the first two blanked subframes but not in the third, unprotected subframe. The pico base station may also consider scheduling some non-cell edge users in the third, unprotected subframe. The ABS pattern can be sent to other cells, like the pico cell in FIG. 4, which can use these ABSs to schedule UEs subject to high interference from surrounding cells, e.g., UEs in the cell range extension area. ABS are defined in 3GPP TS 36.331, incorporated herein by reference, which describes transfer of ABS patterns over the LTE X2 interface between eNBs as bit strings 40 bits long for frequency division duplex (FDD), and 70 bits for time division duplex (TDD). Each bit corresponds to one subframe (SF).
A second interference reduction approach uses Relative Narrowband Transmit Power (RNTP) signaling, which indicates whether the average transmitted power per physical resource block (PRB) in the frequency domain will be below a threshold—an RNTP threshold. A macro base station signals the RNTP PRB pattern to inform other cells about those PRBs having reduced DL interference. As with ABS pattern communication, a pico cell can schedule UE transmissions from the cell edge or UEs in the cell range extension area using lower power PRBs. This reduces UL interference in the macro cell.
In a third interference reduction approach, if a cellular network operator has several frequency carriers available, the operator can use one carrier for its Pico cells and another carrier for its Macro cells, and thereby avoid inducing interference from the macro cell to UEs served by the pico. But if the operator wants to use all or most of the carriers for its macro network, then it can be helpful to know to investigate to what extent the carrier reserved for the pico can be used also by the macro base station.
In the non-limiting example scenarios described below, the macro cell is the aggressor, and the victim UEs are served by the pico cell. This can be generalized to scenarios where a different cell type is the aggressor inducing excessive interference to a victim UE served by a different cell type. One such example is when the pico cell implements a restricted access to services, also referred to as closed access, which means that some UEs in the pico service area, but served by a macro cell, will be victim UEs while the pico cell is the aggressor.
There are a number of problems with these three approaches to inter-cell interference reduction. First, none of the approaches determines, which pattern of subframes should be blanked for ABS use and which RPSF subframe patterns to use low power on. ABS can be transmitted at zero or reduced power, which means that the RPSF pattern can be seen as an ABS pattern extended with reduced power level information. But it is not possible to configure the reduced power information in the existing 3GPP specifications. Second, even though 3GPP TS 36.300 specifies that the OaM takes that responsibility, there is currently no specification for the OaM to configure these ABS/RPFS patterns. Nor is there a way to ensure that the same ABS/RPFS pattern is used on different cells. But it would be desirable to ensure that the same pattern is used on different cells so that the Macro eNBs in a given neighbourhood allocate the same ABS pattern for its cells. The “protected” subframes in that pattern may then be used for highly interfered UEs to reduce interference. In other words, if a cell serves UEs that are interfered by more than one other cell, then with time-aligned ABS from these other cells, it is possible for the cell with the interfered UEs to schedule those UEs during subframes with less interference from other cells according to the ABS pattern. Third, there is no provision for an OaM to independently configure these patterns or to take feedback into account. The current 3GPP specifications mention reporting usefulness at an eNB of an ABS pattern from a neighboring eNB, but nothing is specified, either at the RAN level or at the OaM level. A fourth problem is that there is currently no way to determine or evaluate if a current configuration of ABS/RPSF is actually effective to meet interference reduction needs.